Oftentimes, an optional component part such as a switch, a loudspeaker and a cup holder is required to be attached to an upholstery member such as a lining (trim) or an instrumental panel that defines an inner wall of a vehicle passenger compartment. Conventionally, when attaching an optional component part onto a lining, a part of the lining is cut out, and the optional component part is fitted into the cutout and fixedly secured therein (see patent document 1, for instance). It is also known to use self-tapping screws to directly secure the optional component part to the lining.